Pusher change history: Elevated API Errors in AP4 Cluster

Pusher reported a major AP4 incident affecting Channels API message publishing, with elevated errors and latency, starting 2025-10-21 15:06 UTC and resolved at 2025-10-21 17:43 UTC (computed duration: 2 h 36 min). Pusher identified an issue with a backend caching server and implemented a fix; many customers may have been unable to publish new messages.